You know full well that trades entered before the close can complete after the close. Period, end of story. Have enough of them stacked up right at the close and it could take a while to print them. Nothing strange or unusual here, unless perhaps a BUNCH of longs decided at 3:59:50 to not carry this burden any longer.
"’cause ev’ry stock's a winner and ev’ry stock's a loser,
And the best that you can hope for is to dump them on some sheep. - chisox pat, The (Urban) Gambler